Devices for monitoring, measuring, or diagnosing a physiological condition or a biological phenomenon are known in the art. Some of these devices are able to quickly and/or non-invasively evaluate a condition or detect a phenomenon by using spectrophotometry. For example, a number of procedures for monitoring or diagnosing medical conditions benefit from the ability to use spectrometric means to accomplish the procedure. An example is pulse oximetry. As will be appreciated by one of ordinary skill in the art, the degree of oxygen saturation of hemoglobin, SpO2, in arterial blood is often a vital index of the condition of a patient. As blood is pulsed through the lungs by action of the heart, a certain percentage of the deoxyhemoglobin, Hb, oxygenates so as to become oxyhemoglobin, HbO2. From the lungs, the blood passes through the arterial system until it reaches the capillaries at which point a portion of the HbO2 delivers its oxygen to support the life processes in adjacent cells.
By medical definition, the oxygen saturation level is the ratio of HbO2 to the total hemoglobin; therefore as will be appreciated, SPO2=HbO2/(Hb+HbO2). The saturation value is a significant physiological value. A healthy, conscious person will have an oxygen saturation of approximately 96 to 98%. A person can lose consciousness or suffer permanent brain damage if that person's oxygen saturation value falls to very low levels for an extended period of time. Because of the importance of the oxygen saturation value, pulse oximetry has been recommended as a standard of care for every general anesthetic.
As stated, some pulse oximeters use spectrophotometry to determine the saturation value of blood. Specifically, these oximeters analyze the change in color of the blood to determine the saturation value. As will be appreciated, when radiant energy passes through a liquid and/or tissue, certain wavelengths may be selectively absorbed by particles which are dissolved therein. For a given path length that the light traverses through the liquid, Beer's law (the Beer-Lambert or Bouguer-Beer relation) indicates that the relative reduction in radiation power (P/Po) at a given wavelength is an inverse logarithmic function of the concentration of the solute in the liquid that absorbs that wavelength.
For a solution of oxygenated human hemoglobin, the absorption maximum is at a wavelength of about 640 nanometers (red), therefore, instruments that measure absorption at this wavelength are capable of delivering clinically useful information concerning oxyhemoglobin levels.
In general, methods for non-invasively measuring oxygen saturation in arterial blood utilize the relative difference between the electromagnetic radiation absorption coefficient of deoxyhemoglobin, Hb, and that of oxyhemoglobin, HbO2.
It is well known that deoxyhemoglobin molecules absorb more red light than oxyhemoglobin molecules, and that absorption of infrared electromagnetic radiation is not affected by the presence of oxygen in the hemoglobin molecules. Thus, both Hb and HbO2 absorb electromagnetic radiation having a wavelength in the infrared (IR) region to approximately the same degree; however, in the visible region, the light absorption coefficient for Hb is quite different from the light absorption coefficient of HbO2 because HbO2 absorbs significantly more light in the visible spectrum than Hb.
In the practice of the typical “pulse oximetry” technique, the oxygen saturation of hemoglobin in intravascular blood is determined by (1) alternatively illuminating a volume of intravascular blood with electromagnetic radiation of two or more selected wavelengths, e.g., a red wavelength and an infrared wavelength, (2) detecting the time-varying electromagnetic radiation intensity transmitted through or reflected by the intravascular blood for each of the wavelengths, and (3) calculating oxygen saturation values for the patient's blood by applying the Lambert-Beer's transmittance law to the detected transmitted or reflected electromagnetic radiation intensities at the selected wavelengths.
As will be appreciated from the foregoing, whereas apparatuses are available for making accurate measurements on a sample of blood in a cuvette (in vitro), these devices suffer from the drawback that they do not permit in vivo, in situ, analysis. As it is not always possible or desirable to withdraw blood from a patient, and it obviously is impractical to do so when continuous monitoring is required, such as while the patient is in surgery. Therefore, much effort has been expended in devising instrumentation for making such measurements by noninvasive or less invasive means.
The pulse oximeters used today are desk-top models or handheld models that are interfaced to the patient through the use of a multi-wire bundle. Despite their seemingly manageable size and the sophistication of technology, these units are still bound by several limitations. For example, as will be appreciated by one of ordinary skill in the art, these devices are still too big and unwieldy for use in monitoring smaller vessels and areas of circulation. Also, as will be appreciated, these prior art devices lack the minute size and ability to be coupled to a particular area of a patient for continuous monitoring of a precise area or condition in, for example, a trauma situation.
The foregoing and like drawbacks are also limiting on other monitoring and diagnosing devices and methods. For example, presently, hand-held Doppler ultrasound is used to monitor surgical flap integrity. As will be appreciated, ultrasound is useful only in monitoring larger arteries and is ill adapted to give information about the circulation at the capillary bed level. Likewise, surgical flaps and grafts often have tenuous blood supplies which make it very difficult to monitor them effectively with the currently available devices.
Moreover, the use of spectrophotometric monitoring of deep tissues and organs for oxygenation, indocyanine green clearance and other like spectrophotometric phenomenon is, at best, difficult and cumbersome with the currently available devices.
The foregoing underscores some of the problems associated with conventional diagnosing and monitoring devices. Furthermore, the foregoing highlights the long-felt, yet unresolved need in the art for a portable and reliable device adapted to allow a user to monitor a particular area of interest, even if at the capillary bed level or even if having a tenuous blood supply, with isolated or continuous spectrophotometric means.